Are you getting a when you try to send a message, or is it just failing silently?
If your carrier has blocked your number due to spam or abuse, you'll need to contact them to resolve the issue. Here's how: how to remove message blocking on iphone
Contact your carrier's customer support or log into your cellular account app to check for any active "Block" features on your line. 4. Refresh Your Network Settings Are you getting a when you try to